This invention relates to dynamic expansion and, more particularly, to a dynamic expander which substantially restores the dynamic range of a time-varying signal without adding perceptible distortion.
Dynamic expanders are known in the art and are used to restore the energy distribution of the original program material which is lost due to compression in the recording process. That is, in the recording of program material, particularly musical material, high volume passages are deliberately limited in loudness and otherwise compressed in order to prevent saturation in the master tape. Another reason for this compression is to limit the program material to facilitate pressing of phonographic record discs. This is done to prevent the record groove requirement from becoming excessively wide or even exceeding the physical limitations of the record grooves. The problem arising from this requirement for compression is further aggravated in that low-level passages, which would otherwise be masked in the recording equipment system noise, are in many cases increased in amplitude to improve the signal-to-noise ratio. This increase in amplitude of low-level passages acts to further and undesirably limit the overall dynamic range of the original program material and accordingly, limits the full impact of the original material.
It is therefore the desired function of a dynamic expander to compensate for the compression thereby to restore the original dynamic range and to do so without adding distortion or "coloration" to the original program material. In the prior art, attempts have been made to control the gain of one or more amplifiers of a dynamic expander in order to restore the original dynamic range of the program material. It is known in the prior art that any gained control amplifier typically has, by definition, distortion producing mechanisms largely due to the non-linear devices employed to vary the amplifier gain. Early prior art dynamic expanders nevertheless utilized such distortion producing mechanisms and therefore typically exhibited a characteristic distortion or coloration including distortion commonly known as "breathing" or "pumping". This breathing or pumping is an audible affect wherein both thumping and wide, fluctuating noise level variations accompany the operation of the expander as expansion occurs. Moreover, the use of non-linear control devices and relatively fast gain control signals resulted in intermodulation (IM) between the program material signal and the control signal which, particularly at low program signal frequencies, created very objectionable distortion.
As the prior art developed, the trend to eliminate distortion and breathing was (and still is) to use faster and faster attack times and relatively fast decay times in the circuitry which controls the gain of the amplifiers of the dynamic expander. As this trend developed, the attack and decay times of the gain control signals approached impulse functions relative to the average time period or frequency of the program signal. It is known from information theory, however, that sidebands, intermodulation (IM) and other distortion products are created by such impulse functions. That is, if gain is nearly instantaneously changed (which is a result of faster and faster attack times) sidebands and distortion products occur at the speed of the attack. Consequently, the sidebands and distortion products, which are characteristic of the prior art, normally fall within the audible passband of the program signal and therefore can be heard. This is a problem. Moreover, present day dynamic expanders exhibit distortion due to non-linear gain control; distortion due to intermodulation (IM) between the control signal and the program signal; distortion caused by the intermodulation of the control signal and its own signal envelope; breathing due to the rapid change between expansion and non-expansion modes; and, breathing due to the sidebands created by the fast changing impulse-like control signals.
Another prominent disadvantage of present day dynamic expanders is their limited range which is on the order of 3 to 6 db of maximum obtainable expansion. The maximum expansion is limited to this range because, previous to the invention described hereinafter, in order to exceed this range the resulting distortion and coloration becomes too objectionable.
In the dynamic expander of the present invention, however, these and other disadvantages are overcome by using a technique which is a radical departure from, and contrary to the teachings of, the prior art.